text: The word intron is derived from the terms intragenic region, and intracistron, that is, a segment of DNA that is located between two exons of a gene. The term intron refers to both the DNA sequence within a gene and the corresponding sequence in the unprocessed RNA transcript. As part of the RNA processing pathway, introns are removed by RNA splicing either shortly after or concurrent with transcription. Introns are found in the genes of most organisms and many viruses. They can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA).
  Within introns, a donor site (5 end of the intron), a branch site (near the 3 end of the intron) and an acceptor site (3 end of the intron) are required for splicing.  The splice donor site includes an almost invariant sequence GU at the 5 end of the intron, within a larger, less highly conserved region. The splice acceptor site at the 3 end of the intron terminates the intron with an almost invariant AG sequence.  Upstream (5-ward) from the AG there is a region high in pyrimidines (C and U), or polypyrimidine tract.  Further upstream from the polypyrimidine tract is the branchpoint, which includes an adenine nucleotide involved in lariat formation. The consensus sequence for an intron (in IUPAC nucleic acid notation) is: G-G-[cut]-G-U-R-A-G-U (donor site) ... intron sequence ... Y-U-R-A-C (branch sequence 20-50 nucleotides upstream of acceptor site) ... Y-rich-N-C-A-G-[cut]-G (acceptor site). However, it is noted that the specific sequence of intronic splicing elements and the number of nucleotides between the branchpoint and the nearest 3’ acceptor site affect splice site selection. Also, point mutations in the underlying DNA or errors during transcription can activate a cryptic splice site in part of the transcript that usually is not spliced. This results in a mature messenger RNA with a missing section of an exon. In this way, a point mutation, which might otherwise affect only a single amino acid, can manifest as a deletion or truncation in the final protein.